7. Project Summary/Abstract Of the 2.6 million U.S. children under age 3 living at or below the Federal Poverty Threshold, most do not receive services to support healthy development for which they are eligible, because of limited program capacity. Existing evidence-based interventions designed to buffer low-income children in this age range from the negative effects of stressors associated with poverty require high dosage and/or long-duration delivery from trained professionals to achieve only modest effect sizes, and they have high rates of attrition. Moreover, very little is known about mediators and moderators of the effectiveness of these interventions (i.e., what works for whom and why). This is especially true in terms of potentially malleable caregiver and child neurobiological mechanisms that represent mediators of program impact. The overall objective of the proposed study is to conduct a randomized effectiveness trial with a diverse sample of low-income families with children ages 12? 36 months, and who are eligible for Early Head Start (EHS) but cannot be enrolled because of limited program capacity. We will use a longitudinal randomized effectiveness trial to test the central hypothesis that associations between increases in responsive caregiving (the main FIND target), and subsequent caregiver well-being and child developmental and biobehavioral outcomes (secondary targets), will be partially mediated through changes in caregiver neuroimaging-based and behavioral measures of inhibitory control and parent self-concept. We will also examine moderators of hypothesized intervention effects. The rationale for this work is that it simultaneously addresses the unmet needs of a large, significantly underserved early childhood population and allows for a rigorous test of our conceptual model. We will randomize 300 primary caregivers and their 1- to 3-year-old children who are eligible for EHS services but who cannot be served, to receive FIND or an active control intervention. Aim 1 quantifies the main effects of FIND on changes in responsive parenting and related caregiver and child outcomes (including caregiver-reported parenting self-efficacy, objective measures of chronic child stress) immediately after the intervention, and the durability of these effects 6 months later. Aim 2 identifies underlying neural mechanisms that mediate associations between FIND-related changes in caregiver behavior and caregiver/child outcomes and the specificity of those mechanisms. Aim 3 assesses the degree to which these associations are moderated by caregiver and child characteristics (e.g., caregiver history of adversity, family socioeconomic status, child age) and intervention fidelity and dosage. This information is critical to addressing differential response to early childhood interventions for children ages 1?3, to increase impact and scalability. These outcomes will have a positive impact in that the overwhelming majority of families who meet the enrollment criteria for EHS normally do not receive services. An easily delivered program that can be provided while these families remain on the waitlist may facilitate their children's development and reduce risks.